When a well is drilled and steel casing is placed, cement slurry is pumped into the annular space between casing and formations. The primary objectives of cementing are to provide mechanical support for the steel casing string and to provide zonal isolation between earth strata or formations, i.e. to keep the different zones or strata in the subsurface hydraulically isolated from each other as they were before the well was drilled. Multiple-stage casing and cementing operations are common procedures to establish pressure barriers during drilling a well. It allows the use of heavier drilling muds in drilling deeper sections without damaging or fracturing the shallower formations due to hydrostatic pressure gradient. An ideal cementing job would fill the casing and formation annulus completely with cement. Potential issues often encountered in cementing operations are fluid filled channels within the cement sheath and fluid contaminated cement due to incomplete replacement or sweeping of drilling mud with cement slurry. Zonal isolation assessment is a critical aspect of well integrity tests to ensure hydrocarbon production in a safe manner. Cement evaluation measurements are relied upon to prevent fluid cross flows from unwanted zones other than the producing intervals.
This invention relates to in situ evaluation of cement quality between steel casing and formations in a wellbore. More particularly, this invention relates to a method for in situ detection of fluid channels behind the casing to provide quantitative measurement of cement slurry and fluid volumes. A fluid-filled channel in the cement may allow fluid flows between formation zones and eventually compromise well integrity.
Cement quality surveys are traditionally conducted downhole by running cement bond logging tools in a cased wellbore. Acoustic wave propagation phenomena are exploited to extract physical properties of the media with which the wave interacts. A typical cement bond tool consists of one of more acoustic transducers to emit pressure waves and one or more acoustic receivers to record the returning waveforms. Cement bond logging (“CBL”) technology is often loosely categorized as one of the cased hole logging technologies, and it is more closely related to sonic or acoustic logging. Most of the sonic logging tools are designed with an operational mode for CBL measurements in addition to compressional and shear wave as well as other modes.
Cement evaluation techniques have traditionally involved the use of acoustic (sonic and ultrasonic) measurements to infer presence of a cement bond to casing and detect fluid channels if no cement bond is present. The common practice is to make a logging pass while the casing is under hydrostatic pressure only (zero pressure) and to subsequently run another pass while casing is pressurized in order to close out micro annulus between casing and cement. Micro annulus refers to the gap of several microns which often leads to poor bond logs. The CBL measurement principles are based on acoustic wave propagation theory. Cement bond logging tools consist of an acoustic transducer and receiver pairs. Either acoustic amplitude or impedance (“AI”) is extracted from the recorded acoustic waveforms and are used to infer bonding between cement and casing and distinguish cement from mud. In recent years, many wells drilled in a low fracture gradient or highly permeable formations are completed with light weight and foam cements to minimize formation damage. These types of cements are difficult to evaluate using traditional acoustic impedance measurements as light weight cements have acoustic properties similar to drilling muds.
Following is a listing of a few selected papers on cement bond evaluation using traditional acoustic logging methods:    Goodwin, K. J., “Guidelines for ultrasonic cement-sheath evaluation,” SPE 19538, 1992.    Havira, R. M., “Ultrasonic cement bond evaluation,” 1982 SPWLA annual logging symposium, paper N.    Leigh, C. A., “Results of field testing the cement evaluation tool,” 1984 SPWLA annual logging symposium, paper H.    Butsch, R. J., “The evaluation of specialized cements,” SPE 76713.    Grosmangin, M., “A sonic method for analyzing the quality of cementation of borehole casing,” SPE 1512-G, 1961.    Harness, P. E., “New technique provides better low-density-cement evaluation,” SPE 24050, 1992.    Rao, N., “Parametric study of cement bond evaluation using early refracted arrivals,” 1997 SPWLA annual logging symposium, paper P.
A partial summary follows of other papers on cement evaluation or related well logging techniques.
U.S. Pat. No. 8,100,177 (“Method of logging a well using a thermal neutron absorbing materials”) discloses a method to detect formation fracture location and height after a wellbore is hydraulically fractured and fluids loaded with proppants are injected into the fracture openings. The proppants are doped with thermal neutron absorbers that can be detected using a pulsed neutron logging tool. It is not applicable for cement evaluation because the fracture openings extend further beyond the cement sheath.
The disclosure in U.S. Pat. No. 3,815,677, “Method for operating in wells,” is related to running an open hole sonic log and cased hole neutron log to detect fluid channels in cement. Running an open hole neutron log is not disclosed. In more detail, the method disclosed in this patent consists of running neutron logs in wells completed with two or more production tubing strings. Neutron logs are acquired in each of the tubing strings separately and then are compared to identify response anomalies caused by fluid channels near one of the tubing strings and whether or not the two logs depart from each other. Neutron logs would read higher porosity values in fluid filled channels than in cement. The method also uses an open hole sonic log which provides open hole porosity measurement similar to neutron porosity. In situations where channels are present around both of the tubing strings, it may be difficult to distinguish channels from cement as neutron porosity values would be very close to each other, and the open hole sonic porosity log would provide a reference porosity log.
PCT International Patent Application Publication WO 2012/036689 (“Combined sonic/pulsed neutron cased hole logging tool”) discloses a method for using sonic and pulsed neutron logs simultaneously to evaluate rock properties and cement integrity. It relates to formation and casing evaluation tools and methods of formation valuation, and more particularly a combination of sonic and pulsed neutron tool for formation evaluation through casing and casing and cementing integrity evaluation and methods for using the same. It does not teach obtaining an open hole neutron log, and provides an alternative through a casing formation evaluation tool to open hole formation evaluation tools by combining pulsed neutron and sonic technology in a mono-cable format for use in a single cased hole logging run. It is also an efficient and cost saving approach to obtaining the desired formation evaluation data, such as fluids saturations and rock properties of the reservoir including compressional and shear slownesses, minimium horizontal stress profile, porosity, simple mineralogy, matrix sigma, pseudo formation density, and full wave information for well design and hydraulic stimulation design and for use in analyzing casing and cement integrity.
SPE paper 35681 (“Neutron logs improve interpretation of foamed cement, even in concentric casing”) was presented at the 1996 SPE Western Regional Meeting. It describes a method for using both open hole and cased hole neutron logs to evaluate foam cement qualitatively. Open and cased hole neutron logs are first environmentally corrected by removing the casing and cement effects. The ratio of the open and cased hole logs is then obtained and used as a cement quality indicator. A potential problem with this method is that the amounts of casing and cement effect corrections cannot be determined. The amounts of correction in cement correction charts assumes a perfect cement condition. Furthermore, taking the ratio of the open and cased hole logs reduces the dynamic range of the measurement sensitivity.
Thus, there is a need for a method for well cement testing that uses a non-acoustic logging tool and is better able to evaluate and detect fluid channels in light weight and foam cements.